Evaporation is a natural phenomenon and a form of energy transfer in the Earth's climate. Nevertheless, evaporation as a source for powering engineered systems has not been fully explored. Nanoscale confinement of water in hygroscopic materials (i.e., objects that take up and retain moisture) can provide a way to convert energy from evaporation by generating mechanical force in response to changing relative humidity. However, scaling up this phenomenon to create macroscopic devices can face challenges: unfavorable scaling of hydration kinetics can slow down actuation speeds at large dimensions; small strains can complicate energy transfer to external systems; and the slow rate of change of relative humidity in the environment can limit the power output.
While evaporation carries a significant energy, it can involve a slow rate of water transfer that can limit the relative expansion and contraction of hygroscopic materials. Because the relative volume of the absorbed and released water can be small, the pressure change generated during this process should be large for efficient energy conversion. Water confined to nanoscale cavities within hygroscopic materials (FIG. 1a) can induce large pressures in response to changing relative humidity; however, these nanostructures can also limit the transport kinetics of water. Simply scaling up the dimensions of hygroscopic materials would not increase power, and can even lead to a decrease, because the time scale of wetting and drying can depend on the square of the travel distance of water.
Addressing the issues in transport kinetics alone would not necessarily be sufficient to increase power, because in certain environmental conditions relative humidity changes on daily and seasonal timescales, which can be too slow. However, spatial gradients in relative humidity established near evaporating surfaces can provide an opportunity.